The present invention relates generally to gear boxes, and more particularly, to a two speed gear box for driving a hub, such as a wheel hub assembly.
In automotive design, the placement of the engine and the transmission is highly constrained by the function of the vehicle and its weight distribution, the size of the vehicle, traction considerations, and the position of the drive axle(s). In the case of passenger vehicles, comfort and convenience for the passengers are important considerations. One particular design constraint for off road vehicles is the requirement for all-wheel drive. On heavy equipment applications, such as all-wheel drive earth moving vehicles, the placement of the engine, the transmission, and the drive system components are particularly critical design considerations.
For example, on earth moving vehicles, poor ground conditions typically cause poor traction between the tires and the ground. To increase useful traction, such vehicles are typically designed as all-wheel drive vehicles. However, one drawback to using all-wheel drive systems on earth moving vehicles is that conventional placement of the engine, the transmission and the drive axle may interfere with the operation of the vehicle. For example, a tractor scraper, which self loads soil material and hauls the soil material to an unloading area, often encounters poor traction conditions. In order to properly cut and load the soil, a tractor scraper must have adequate traction at low speed. Adequate traction is typically achieved by using an all-wheel drive system during the loading cycle. However, in order to be effective as a haulage vehicle, a tractor scraper should also have high speed on-road capabilities.
The arrangement of a tractor scraper, with the bowl for the soil payload disposed between the drive axles, does not easily permit placement of a mechanical drive line between the two drive axles so as to permit driving force to be transmitted to all of the drive axles from a single engine. As a result, all-wheel drive may often be achieved by provided two engines and two mechanical transmissions, each driving a corresponding one of the two drive axles.
As an alternative to a mechanical drive system, hydrostatic drive systems are often employed for vehicles such as tractor scrapers. Hydrostatic drive systems are ideal transmission mechanisms for tractor scrapers because such drive systems are capable of transmitting power from one power source to multiple locations. However, the torque and speed range of hydrostatic drive systems are limited compared to those of mechanical transmissions.